=Teeth and toes= are the basis for subdividing the class mammalia into orders. Although the breathing, the circulation, and the internal organs and processes are similar in all mammals, the external organs vary greatly because of the varying environments of different species. The internal structure enables us to place animals together which are essentially alike; _e.g._ the whale and man are both mammals, since they resemble in breathing, circulation, and multiplication of young. The external organs guide us in separating the class into orders. The teeth vary according to the food eaten. The feet vary according to use in obtaining food or escaping from enemies. This will explain the difference in the length of legs of lion and horse, and of the forms of the teeth in cat and cow. Make a careful study of the teeth and the limbs as shown in the figures and in all specimens accessible. Write out the dental formulas as indicated at the top of page 194. The numerals above the line show the number of upper teeth, those below the line show the number of lower teeth, in one half of the jaw. They are designated as follows: _I_, incisors; _C_, canine; _M_, molars. Multiplying by two gives the total number. Which skulls in the chart have the largest canines? Why? The smallest, or none at all? Why? Compare the molars of the cow, the hog, and the dog. Explain their differences. In which skulls are some of the molars lacking? Rudimentary? Why are the teeth that do not touch usually much smaller than those that do? [Illustration: FIG. 343.—POLAR BEAR (_Ursus maritimus_). ] KEY, OR TABLE, FOR CLASSIFYING MAMMALS (_class Mammalia_) INTO ORDERS │ ORDERS │ =A_{1} Imperfect Mammals=, young hatched │ or prematurely born │ │B_{1} Jaws a birdlike beak, _Mon’otremes_ │ egg-laying │B_{2} Jaws not beaklike, young _Marsu’pials_ │ carried in pouch =A_{2} Perfect Mammals=, young not │ hatched, nor prematurely born │ B_{1} │C_{1} Front part of both jaws _Eden’tates_ _Digits │ lack teeth with │ claws_ │ │C_{2} Teeth with sharp points _Insect’ivors_ „ │ for piercing shells of │ insects │C_{3} Canines very long, _Car’nivors_ „ │ molars suited for │ tearing „ │C_{4} Canines lacking, _Rodents_ │ incisors very large B_{2} │C_{1} Head large; carnivorous _Ceta’ceans_ _Digits │ not │ distinct_│ „ │C_{2} Head small; herbivorous _Sire’neans_